(1) Field of the Invention
The invention relates to a levelling apparatus having a lower part and an upper part which is mounted to the lower part and which is adapted to be adjusted relative to the lower part. The lower part has reference elements for its definite positioning. The upper parts equipped with means for mounting a workpieceholder for a workpiece to be operated upon by a machine tool especially a wire erosion machine.
(2) Description of Related Art
Such a levelling apparatus is disclosed in the document U.S. Pat. No. 5,769,561. The workpieceholder is mainly a vise which is adapted to hold a workpiece to be cut by an erosion wire. An example of such a vise is disclosed in EP-B-196,544 (=U.S. Pat. No. 4,925,168).
Modern wire cut erosion machines cut more precisely and more efficiently than older machines of such type. This improvement is achieved by a higher electric current flow and an improved control of the current flow and an improved removal of waste by an elevated pressure of the rinsing fluid. The nozzle through which the rinsing fluid emerges under high pressure, is positioned in most of said machines in close proximity to the workpiece to be worked upon. The pressure of the rinsing fluid is not constant but varies in short intervals of time. Such variations do not impair the rapid removal of waste but have an impact on the definite position of the workpiece. The workpiece is induced to vibrate and vibrations of the workpiece caused thereby can hardly be suppressed by the known levelling device. Thus, the workpiece suffers from deviations from its definite position and an accurate cutting of the workpiece can hardly be achieved.
It is therefore an object of the invention to devise an improved levelling apparatus which ensures rigidity and sturdiness of the workpiece positioning close to the cutting wire even if the pressure of the rinsing fluid varies.
To this end the invention provides for a levelling apparatus which has a lower part and an upper part mounted to the lower part and adapted to be adjusted with respect to the lower part, wherein the lower part is provided with reference elements for its definite positioning, and wherein the upper part is provided with means for mounting a workpieceholder for a workpiece which is to be worked on by a machine tool especially a wire erosion machine, and wherein the lower side of the upper part has a first beam structure and the upper side of the lower part has a second beam structure which is complementary to the first beam structure such that the beams of the first beam structure extend between the beams of the second beam structure. The invention allows a substantial improvement of the rigidity and sturdiness of the levelling apparatus without the necessity to enhance the thickness of the upper part and of the lower part. If the thickness of the upper part and of the lower part would be augmented it would no longer be possible to position the workpiece very close to the exit of the cutting wire of a wire erosion machine in view of the restricted space left free for the workpiece. On the other hand it is mandatory to place the workpiece as close to the cutting wire exit as possible for obtaining high precision cuts.
According to a preferred embodiment of the invention the beams of the first beam structure and the beams of the second beam structure are parallel and straight. Preferably, the beams of the first beam structure are directed towards the means for mounting the workpieceholder. The rigidity is further improved if according to an embodiment of the invention the height of each beam of the first beam structure measures at least one third of the width of such beam. It is also advantageous with respect to rigidity if at least some of the beams of the first beam structure are connected by transverse beams. Thereby a type of a beam lattice work is obtained.
Furthermore, the means for mounting the workpieceholder may include an extension form from a front edge of the upper part wherein the extension may be connected to the lower side of the upper part by reinforcing bars. Such reinforcing bars may extend into spaces between beams of the first beam structure. Moreover, the reinforcing bars may be inclined downwardly and forwardly in view of an improvement according to which the extension extends downwardly towards a lateral side of the lower part, specifically if the upper part is mounted on the top of the lower part.
According to an additional improvement of the invention a space is formed between neighboring beams of the first beam structure, the space having a width which is substantially equal to the width of one of the laterally adjacent beams. The second beam structure may comprise two opposite lateral beams extending outside and adjacent to the upper part and, preferably, transversely to a front edge of the upper part. It is useful to make the lateral beams run parallel to each other. In adaption to the inclination of the reinforcing bars the second beam structure may comprise intermediate beams each having a front ramp.
The exact positioning of the workpiece clamped to the workpieceholder is made easier if according to another prefered improvement of the invention a first adjustment screw is arranged with great distance to the front edge of the upper part for an up or down adjustment of the upper part relative to the lower part, an axis about which the upper part may be adjusted running closely and parallel to said front edge. Specifically, said axis may be defined by an imaginary connecting line drawn through a bolt which is surrounded by a spring assembly producing a bias on the upper part and the lower part, and through a second adjustment screw for a tilting adjustment of the upper part relative to the lower part. Structurally, the axis may be formed by a first rib which extends parallel to said front edge and projects out of the upper side of the lower part or out of the lower side of the upper part. Moreover, a second rib may be provided which extends transversely to the front edge and projects out of the lower side of the upper part or out of the upper side of the lower part, for tilting the upper part relative to the lower part to the right or to the left. A composite adjusting movement of the upper part therefore will take place about a pivot point which is situated in the crossing of both ribs.
Last not least a specifically preferred embodiment of the invention resides in the provision of a leaf spring of T form which extends between the upper part and the lower part and is mounted either to the upper part or to the lower part, a transverse leg of the leaf spring being spaced to the front edge and being fastened to the upper part or to the lower part, and a longitudinal leg of the leaf spring being directed to the front edge and being fastened to a middle beam of the first beam structure or of the second beam structure. Moreover, the leaf spring has a front end which is close to the front edge and extends between the first rib and the second rib. Thus, the rib of the upper part will rest upon the front end and the front end will rest upon the rib of the lower part.
According to a further aspect of the invention free surfaces of the apparatus may be covered by vibrations damping material.